abd_iter_page: don't use compound heads on Linux <4.5

[mirror_zfs.git] / module / os / linux / zfs / abd_os.c

diff --git a/module/os/linux/zfs/abd_os.c b/module/os/linux/zfs/abd_os.c
index 3fe01c0b7d77a4be52e1e3158c55073ae65b2eec..d3255dcbc0f78781d6f324b3f4c01d4ddb6cc52d 100644 (file)
--- a/module/os/linux/zfs/abd_os.c
+++ b/module/os/linux/zfs/abd_os.c
@@ -62,6 +62,7 @@
 #include <linux/kmap_compat.h>
 #include <linux/mm_compat.h>
 #include <linux/scatterlist.h>
+#include <linux/version.h>
 #endif
 
 #ifdef _KERNEL
@@ -1061,6 +1062,7 @@ abd_iter_page(struct abd_iter *aiter)
        }
        ASSERT(page);
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 5, 0)
        if (PageTail(page)) {
                /*
                 * This page is part of a "compound page", which is a group of
@@ -1082,11 +1084,23 @@ abd_iter_page(struct abd_iter *aiter)
                 * To do this, we need to adjust the offset to be counted from
                 * the head page. struct page for compound pages are stored
                 * contiguously, so we can just adjust by a simple offset.
+                *
+                * Before kernel 4.5, compound page heads were refcounted
+                * separately, such that moving back to the head page would
+                * require us to take a reference to it and releasing it once
+                * we're completely finished with it. In practice, that means
+                * when our caller is done with the ABD, which we have no
+                * insight into from here. Rather than contort this API to
+                * track head page references on such ancient kernels, we just
+                * compile this block out and use the tail pages directly. This
+                * is slightly less efficient, but makes everything far
+                * simpler.
                 */
                struct page *head = compound_head(page);
                doff += ((page - head) * PAGESIZE);
                page = head;
        }
+#endif
 
        /* final page and position within it */
        aiter->iter_page = page;